diff --git a/src/script/event/preprocessor/EventStorageMiddleware/EventStorageMiddleware.test.ts b/src/script/event/preprocessor/EventStorageMiddleware/EventStorageMiddleware.test.ts
index 710e443f411a..a042642aa733 100644
--- a/src/script/event/preprocessor/EventStorageMiddleware/EventStorageMiddleware.test.ts
+++ b/src/script/event/preprocessor/EventStorageMiddleware/EventStorageMiddleware.test.ts
@@ -20,9 +20,16 @@
 import {Asset as ProtobufAsset} from '@wireapp/protocol-messaging';
 
 import {AssetTransferState} from 'src/script/assets/AssetTransferState';
+import {ConversationState} from 'src/script/conversation/ConversationState';
+import {Conversation} from 'src/script/entity/Conversation';
 import {User} from 'src/script/entity/User';
 import {EventError} from 'src/script/error/EventError';
-import {createAssetAddEvent, createMessageAddEvent, toSavedEvent} from 'test/helper/EventGenerator';
+import {
+  createAssetAddEvent,
+  createMemberLeaveEvent,
+  createMessageAddEvent,
+  toSavedEvent,
+} from 'test/helper/EventGenerator';
 import {createUuid} from 'Util/uuid';
 
 import {EventStorageMiddleware} from './EventStorageMiddleware';
@@ -37,9 +44,15 @@ function buildEventStorageMiddleware() {
     replaceEvent: jest.fn(event => event),
     deleteEvent: jest.fn(),
   } as unknown as jest.Mocked<EventService>;
-
+  const conversationState = {
+    findConversation: jest.fn(),
+  } as unknown as jest.Mocked<ConversationState>;
   const selfUser = new User(createUuid());
-  return [new EventStorageMiddleware(eventService, selfUser), {eventService, selfUser}] as const;
+
+  return [
+    new EventStorageMiddleware(eventService, selfUser, conversationState),
+    {eventService, conversationState, selfUser},
+  ] as const;
 }
 
 describe('EventStorageMiddleware', () => {
@@ -74,6 +87,66 @@ describe('EventStorageMiddleware', () => {
       );
     });
 
+    it('fails for a member leave event when users are not part of the conversation', async () => {
+      const [eventStorageMiddleware, {conversationState}] = buildEventStorageMiddleware();
+      const conversationId = createUuid();
+      const userIds = [createUuid(), createUuid(), createUuid(), createUuid()];
+      const conversation = new Conversation(conversationId, '');
+
+      conversationState.findConversation.mockImplementation(() => conversation);
+
+      const event = createMemberLeaveEvent(conversationId, userIds);
+
+      await expect(eventStorageMiddleware.processEvent(event)).rejects.toEqual(
+        new EventError(
+          EventError.TYPE.VALIDATION_FAILED,
+          'Event validation failed: User is not part of the conversation',
+        ),
+      );
+    });
+
+    it('fails for a member leave event when users are part of the conversation but are deleted already', async () => {
+      const [eventStorageMiddleware, {conversationState}] = buildEventStorageMiddleware();
+      const conversationId = createUuid();
+      const userIds = [createUuid(), createUuid(), createUuid()];
+      const user1 = new User(userIds[0]);
+      const user2 = new User(userIds[1]);
+      const user3 = new User(userIds[2]);
+      user1.isDeleted = true;
+      user2.isDeleted = true;
+      user3.isDeleted = true;
+      const conversation = new Conversation(conversationId, '');
+      conversation.participating_user_ets([user1, user2, user3]);
+
+      conversationState.findConversation.mockImplementation(() => conversation);
+
+      const event = createMemberLeaveEvent(conversationId, userIds);
+
+      await expect(eventStorageMiddleware.processEvent(event)).rejects.toEqual(
+        new EventError(
+          EventError.TYPE.VALIDATION_FAILED,
+          'Event validation failed: User is not part of the conversation',
+        ),
+      );
+    });
+
+    it('does not return an error for a member leave event when users are part of the conversation', async () => {
+      const [eventStorageMiddleware, {conversationState}] = buildEventStorageMiddleware();
+      const conversationId = createUuid();
+      const userIds = [createUuid(), createUuid(), createUuid()];
+      const user1 = new User(userIds[0]);
+      const user2 = new User(userIds[1]);
+      const user3 = new User(userIds[2]);
+      const conversation = new Conversation(conversationId, '');
+      conversation.participating_user_ets([user1, user2, user3]);
+
+      conversationState.findConversation.mockImplementation(() => conversation);
+
+      const event = createMemberLeaveEvent(conversationId, userIds);
+
+      await expect(eventStorageMiddleware.processEvent(event)).resolves.toEqual(event);
+    });
+
     it('fails for a non-"text message" with an ID previously used by the same user', async () => {
       const [eventStorageMiddleware, {eventService}] = buildEventStorageMiddleware();
       const event = createMessageAddEvent();
diff --git a/test/helper/EventGenerator.ts b/test/helper/EventGenerator.ts
index 496c0bd46a85..a9f7c46ea801 100644
--- a/test/helper/EventGenerator.ts
+++ b/test/helper/EventGenerator.ts
@@ -17,11 +17,15 @@
  *
  */
 
+import {MemberLeaveReason} from '@wireapp/api-client/lib/conversation/data/';
+import {CONVERSATION_EVENT} from '@wireapp/api-client/lib/event';
+
 import {AssetTransferState} from 'src/script/assets/AssetTransferState';
 import {
   AssetAddEvent,
   DeleteEvent,
   EventBuilder,
+  MemberLeaveEvent,
   MessageAddEvent,
   ReactionEvent,
 } from 'src/script/conversation/EventBuilder';
@@ -68,6 +72,24 @@ export function createReactionEvent(targetMessageId: string, reaction: string =
   };
 }
 
+export function createMemberLeaveEvent(conversationId: string, userIds: string[]): MemberLeaveEvent {
+  const conversationQualifiedId = {id: conversationId, domain: ''};
+
+  return {
+    conversation: conversationId,
+    qualified_conversation: conversationQualifiedId,
+    data: {
+      qualified_user_ids: userIds.map(userId => ({id: userId, domain: ''})),
+      reason: MemberLeaveReason.USER_DELETED,
+      user_ids: userIds,
+    },
+    from: createUuid(),
+    id: createUuid(),
+    time: new Date().toISOString(),
+    type: CONVERSATION_EVENT.MEMBER_LEAVE,
+  };
+}
+
 export function createDeleteEvent(deleteMessageId: string, conversationId: string = createUuid()): DeleteEvent {
   return {
     conversation: conversationId,